An optoelectronic thread monitor, in which the light beams of a light source are sent via a light guide device to the thread and, after reflection from the thread, via a second light guide device, to a receiver, is known from German patent DE 26,23,856 A1. As soon as the thread is missing at the monitoring point, light beams cease to enter the second light guide device and consequently the receiver as well. After which a signal evaluating device following this will send a signal.
The reliability of operation of this thread monitor is guaranteed as long as the thread does not perform any movement at right angles to the pull-off direction, or the thread is deflected only slightly.
In stitch-forming machines, the needle thread is deflected alternatingly in mutually opposite directions at right angles to the pull-off direction as a consequence of the up and down movements of the needle bar from its middle position. Due to the rotary and oscillating movements of the hook, a movement directed at right angles to the pull-off direction is also transmitted to the hook thread. Irregularities in stitch formation may even bring about a deflection of the needle thread and/or hook thread by an amount equaling several times the "normal" transverse deflection caused by the working movement of the stitch-forming elements. It can therefore be assumed in connection with the use of the above-described thread monitor on stitch-forming machines that the thread will leave the range of action of the thread monitor, at the latest, at the time of onset of an irregularity in stitch formation--if it does not swing out of the range of action of the thread monitors even during the sewing process in cadence with the stitch formation--as a result of which the indication of a thread disturbance, that does not exist in reality, would be brought about. Such a thread monitor is therefore unsuitable for use on such machines.